The present invention relates generally to high performance optical links and, more particularly, to an optical link that utilizes optical carriers transmitted using a single transmission medium such as an optical fiber in a way which substantially eliminates unwanted distortion signals accumulated during the transmission process.
Optical links early in the development of communication systems using a light transmission medium such as, for example, an optical fiber were based on a single optical carrier of a specific wavelength. FIG. 1 illustrates one such example. An optical link, generally indicated by the reference number 10, shown in FIG. 1 utilizes an optical light source 12 of a selected wavelength xcex1 to produce an optical carrier 14. The specific wavelength was chosen to fall within one of the transmissive windows in the typical optical fiber absorption spectrum. An RF signal 16 is modulated onto the optical carrier 14 using a modulator 18. A modulated optical signal 20 is then directed into an optical fiber 22. Optical devices, such as an optical amplifier 24, may be spliced into optical fiber 22. Modulated optical signal 20 is received at the other end of optical fiber 22 by a receiver 26, and, thereafter, the RF signal is regenerated as an RFout, signal 28.
While prior art optical link 10 is generally suited to its intended purpose, a number of disadvantages have been discovered. The main disadvantages of the single wavelength, single fiber optical link are its relatively limited data capacity and significant data signal degradation. This degradation is due at least in part to optical carrier intensity attenuation and/or composite second order (CSO) noise accumulation during transmission through the optical fiber.
One prior art approach in an attempt to improve data transfer uses multiple optical carriers. However, in an optical link, the number of distinct sets of data which can be transmitted through a single optical fiber on multiple optical carriers is limited by the fact that, if portions of the RF signals modulated onto the optical carriers overlap, those portions will cancel each other and result in crosstalk, or significant reductions in the data fidelity. Hence, only a limited number of distinct data can be transmitted using this optical link scheme. Moreover, when optical amplifiers are inserted into the transmission medium in an attempt to alleviate the attenuation problem, the accumulated noise is amplified along with the optical carrier. For this reason, the carrier-to-noise (CNR) ratio is not improved with the addition of optical amplifiers in a single wavelength, single fiber optical link or in a multiple wavelength, single fiber optical link.
Another prior art attempt in resolving the foregoing problems has been to use a modulator having dual outputs directed into two optical fibers. Now referring to FIG. 2, this dual fiber optical link is generally indicated by the reference number 30. In this dual fiber optical link 30, as in previously described single wavelength, single fiber optical link 10, single laser source 12 is used and single optical carrier 14 is directed into a modulator 32. However, optical carrier 14 is split into first and second optical signals 34 and 36 after RF modulation signal 18 is encoded thereon. Modulator 32 is designed such that RF modulation 18 on first optical signal 34 and RF modulation 18 on second optical signal 36 emitted at the output ports 1 and 2, respectively, have a predetermined relationship. For example, the RF modulation on first optical signal 34 and the RF modulation on second optical signal 36 can be manipulated to be 180xc2x0 out of phase at the two output ports of modulator 32. Note that optical signals 34 and 36 at the output ports 1 and 2, respectively, are of the same wavelength while the RF modulation on first optical signal 34 is the exact reciprocal of the RF modulation on second optical signal 36. Therefore, unless optical signals 34 and 36 are transmitted on two separate optical fibers 38 and 40, the RF modulation on optical signals 34 and 36, being exactly out of phase, will cancel out and the encoded information will be lost.
Still discussing dual fiber optical link 30 shown in FIG. 2, at the receiving end of optical fibers 38 and 40, the RF modulation signals, as affected by the noise accumulation during transmission, are recovered from optical signals 34 and 36 by separate detectors 46 and 48, respectively. The CSO on each of the two recovered RF signals is in phase with respect to one another since the noise was accumulated during the transmission process, while the RF signals themselves are 180xc2x0 out of phase, as dictated by the characteristics of modulator 32. Recovered RF signals 50 and 52 are combined using a 180xc2x0 combiner, in which one of the recovered RF signals, as affected by the noise accumulation, is sent through a 180xc2x0 phase compensator (not shown) then added to the other recovered RF signal, also as affected by the noise accumulation, such that the CSO on the two recovered RF modulation signals is now 180xc2x0 out of phase so as to substantially cancel when added. Furthermore, the recovered RF signals themselves are in phase as a result of the 180xc2x0 phase compensator. Thus, the total RF signal at the destination is increased by a factor of two as compared to single wavelength, single fiber optical link 10. As a result, the CNR of dual fiber optical link 30 improves by 3 dB.
Although the CSO problem has been reduced, abovementioned dual fiber optical link 30 has a drawback in that two separate optical fibers 38 and 40 are required for the transmission of each data set. Two fibers may not be available for use between all data sources and receivers, and this fact will add to the infrastructure cost of using the dual fiber approach. With regard to proper operation of dual fiber optical link 30, a further problem is posed in the system architecture because in this case two matching sets of optical components are necessary between the data source and the destination. Such matching sets of optical fibers, optical amplifiers, and receivers may simply be unavailable. Moreover, the cost of building an optical communication infrastructure using this two fiber optical link is nearly twice as the cost of a single wavelength, single fiber optical link.
The present invention utilizes a highly advantageous and heretofore unseen optical link configuration and associated method using a single transmission medium which overcomes the foregoing limitations and disadvantages while still improving the CSO and CNR performance of the optical link.
As will be described in more detail hereinafter, there is disclosed herein a high performance optical link and associated method for transmission of an RF signal using a single transmission medium, such as an optical fiber having opposing ends and configured such that unwanted distortion signals are produced when optical signals pass through the optical fiber. At one end of the optical fiber, the optical link includes means for transmitting first and second optical signals through the optical fiber. The optical signals have been modulated with the RF signal in such a way that a predetermined relationship is produced between the RF signals modulated on the first and second optical signals. The opposing end of the optical fiber is equipped with an arrangement for receiving the modulated first and second optical signals, including the unwanted distortion signals produced during transmission through the optical fiber, and for regenerating the RF signal from the modulated first and second optical signals while causing the unwanted distortion signals to be canceled based on the predetermined relationship.